Nonlinear optics (NLO) is a branch of optics that describes the behavior of light in nonlinear media. The nonlinear media has the characteristic where the dielectric polarization P responds nonlinearly to the electric field E of the light. This nonlinearity is typically only observed at very high light intensities (e.g., values of the electric field comparable to inter-atomic electric fields, typically 108 V/m) such as those provided by pulsed lasers. When applying a laser signal source to a nonlinear media or material, various changes to the applied laser signal can occur. Notably, such changes can affect the frequency or wavelength of the laser signal as it passes through the given nonlinear media. As such, one application for the use of nonlinear materials is to perform frequency or wavelength conversion of laser signal sources (e.g., convert an incident laser signal having one frequency to a different frequency).
One of the most commonly used techniques of frequency conversion for lasers is via frequency doubling or second-harmonic generation. With this technique, a 1064-nm output from Nd:YAG lasers or an 800-nm output from Ti:sapphire lasers can be converted to visible light, with wavelengths of 532 nm (green) or 400 nm (violet), respectively. Frequency-doubling can be carried out by placing a nonlinear medium in a laser beam. While there are many types of nonlinear media, the most common media are crystals. Commonly used crystals are BBO (β-barium borate), KDP (potassium dihydrogen phosphate), KTP (potassium titanyl phosphate), and lithium niobate, for example. These crystals have the necessary properties of being strongly birefringent, of having a specific crystal symmetry and of being transparent for both the impinging laser light and the frequency doubled wavelength. In addition to crystals, many nonlinear semiconductor materials can be employed to perform the conversion.
One problem with conventional frequency converters for lasers is that some portion of the incident light from the laser source is absorbed along the beam path in the nonlinear media. As the laser signal passes through the nonlinear media, the absorbed laser energy turns into heat, which is then dissipated in the media causing thermal gradients to appear along the path of the laser signal. Such thermal gradients can lead to changes of the index of refraction in the nonlinear material and can thus adversely affect the performance of the nonlinear material and consequently, the associated frequency conversion.